Gynoecium development inRubiaceae-Vanguerieae, with particular reference to the “stylar head”-complex and secondary pollen presentation

Abstract

Gynoecium development in taxa of the tribeVanguerieae (Rubiaceae, subfam.Antirheoideae) was studied, using primarily a 2- and a 5-carpellate genus (Keetia andVangueria) as examples. All investigated taxa, characterized by ovaries with a solitary, ± apically inserted, pendulous anatropous ovule per locule/carpel, showed a very similar gynoecium development. Comparisons with other uniovulateRubiaceae (taxa of subfam.Rubioideae) revealed that notwithstanding the place of insertion and orientation of the solitary ovules (apically vs. basally inserted, pendulous vs. erect, anatropous ovules) the gynoecium development follows the same pattern; differing ovule insertion and orientation can easily be explained by the principle of variable proportions. — Since secondary pollen presentation is characteristic for the tribe, particular attention was paid to the development of the conspicuous “stylar head”-complex, defined here as structural unit comprised of pollen presenting organ, “receptaculum pollinis”, plus stigmatic (i.e., receptive) surfaces. It was found that their morphological differentiation, following the same pattern in all investigated taxa, starts at very early stages of floral development. They already had their final shape at a stage when the ovule development in the ovary had just barely started. The fully developed, actual “receptaculum pollinis” is made up of enormously enlarged, ± cylindrical epidermis cells which have peculiar ring-like thickenings in vicinity of and parallel to the outer tangential walls (a “mechanical barrier” preventing pollen tubes from entering?) and a much smaller-celled subepidermal tissue. With regard to shape, size, and exposure of the stigmatic surfaces, the investigated taxa exhibited certain morphological differences. These were found to be correlated with differences in androecium structure (and, ultimately, pollen presentation).